Digital cameras have been widespread, which convert an optical image of a subject into electric image signal through an imaging device, such as a CCD image sensor or a CMOS image sensor. The image signal is converted into digital image data, and is written in a memory or a recording medium. Many of the digital cameras are provided with a flash device for projecting a flash of illumination light to the subject during an exposure, some of which automatically emit the flash light according to the subject brightness in an automatic flash mode.
FIG. 11 shows an example of a program chart used for an automatic exposure control operation in a conventional digital camera, wherein a horizontal axis shows shutter speed (time value TV), a vertical axis shows aperture value (AV) of a stop, and diagonal lines represent exposure values (EV). In this example of conventional digital camera, imaging sensitivity is fixed to be ISO200, and the aperture value and the shutter speed are changed at each shot according to the exposure value calculated based on the subject brightness. Besides, a threshold exposure value below which the digital camera automatically emits a flash light, hereinafter referred to as a low brightness flashing point, is preset at EV8 as converted to ISO100 base. Furthermore, because hand-shake is likely to occur when the shutter speed is slower than 1/60 second (=TV6), a threshold value below which a hand-shake warning is given, hereinafter referred to as a hand-shake warning point, is preset at TV6.
There have been such digital cameras that judge whether a scene to shoot is a backlit scene or not, and flash automatically to the backlit scene, as disclosed for example in Japanese Laid-open Patent Application Nos. Hei 6-148725, 2001-339636 and 2002-333649. As shown in FIGS. 12A and 12B, these cameras have a photometry area 2 that is divided into several segments 2a, and the segments 2a are partitioned into a central zone 4 and a peripheral zone 5. In most cases, a main subject is placed in the central zone 4, and the peripheral zone 5 corresponds to a background. A photometric value is detected from each segment 2a, and mean photometric values of the central zone 4 and the peripheral zone 5 are calculated respectively. The mean photometric value of the central zone 4 is subtracted from that of the peripheral zone 5 to obtain a photometric value difference, and the photometric value difference is used as a rear light judgment value to be compared with a threshold value for rear light judgment.
More specifically, with respect to such a scene as shown in FIG. 13A, the rear light judgment value will be larger than the rear light threshold value, so the scene of FIG. 13A is judged to be a backlit scene, and a flash light is emitted. In order to avoid misjudging a scene having a dark subject in the central zone 4 to be a backlit scene, the rear light threshold value is set at a relatively large value, e.g. Δ2EV.
Such a digital camera has also been brought into the market that has a function to change over its imaging sensitivity according to the subject brightness, so that a dark scene can be shot at a higher shutter speed. In this case, the imaging sensitivity is virtually changed by changing amplification gain of the image signal obtained through the imaging device. This type of digital camera can extend the flash light reaching distance, and shoot a dark scene with a brighter background, so the atmosphere of the dark scene is adequately reproduced. As the scene is shot at a higher shutter speed, hand-shake is reduced. Moreover, such a scene that would conventionally need a flash light can be shot without the flash.
As shown in a program chart of FIG. 14, the digital camera with the sensitivity changing function uses a standard sensitivity, e.g. ISO200, for those scenes where proper exposure values are relatively high, e.g. over EV8 as converted to ISO100 base. For those scenes where proper exposure values are so low that the aperture value should be set at an open aperture and the shutter speed should be slower than the handshake warning point, the imaging sensitivity is automatically changed to ISO400, ISO800 or ISO1600. Since the imaging sensitivity is automatically increased for the dark scene, the low brightness flashing point is preset at a lower value, e.g. EV5 as converted to ISO100 base. This type of digital camera also has the above-described automatic flashing function based on the rear light judgment.
Meanwhile, when a picture is shot under a bright illumination light at night, there may be a higher brightness portion around the main subject 3 in the peripheral zone 5, as shown for example in FIG. 13B. In that case, because a lower brightness portion surrounds the higher brightness portion, the mean photometric value of the peripheral zone 5 will be so low that the rear light judgment value is less than the rear light threshold value, even while the brightness of the main subject 3 is lower than its peripheral higher brightness portion. As a result, the scene is not judged to be a backlit scene even while it is actually a backlit scene. Such a scene as shown in FIG. 13B will be referred to as a low brightness backlit scene. Since the exposure value for the low brightness backlit scene is lower than the low brightness flashing point EV8 of the conventional digital camera with fixed imaging sensitivity, a flash light is automatically emitted in this type of digital camera, so the main subject of the obtained image will be properly exposed.
However, as the low brightness flashing point is set at the lower value, i.e. EV5, in the digital camera with the sensitivity changing function, if the low brightness backlit scene is so dark that its exposure value is EV8 to EV5, the scene is shot without the flash in this type digital camera, so the obtained image is improperly exposed.
To solve this problem, it has been suggested, for example in the above-mentioned Japanese Laid-open Patent Application No. 2001-339636, lowering the rear light threshold value to promote the automatic flashing to the backlit scenes, while lowering the volume of the flash light. Thereafter, the image quality is corrected in the image processing. If, however, the subject distance is so far that the flash light does not reach the subject, the image processing can result in breaching out the background.